Cypress CY14E256L - Manual (Page 5)

CY14E256L
Document Number: 001-06968 Rev. *F Page 5 of 18
Data Protection
The CY14E256L protects data from corruption during low
voltage conditions by inhibiting all externally initiated STORE
and WRITE operations. The low voltage condition is detected
when V
CC
is less than V
SWITCH
. If the CY14E256L is in a WRITE
mode (both CE and WE are low) at power up after a RECALL or
after a STORE, the WRITE is inhibited until a negative transition
on CE or WE is detected. This protects against inadvertent writes
during power up or brown out conditions.
Noise Considerations
The CY14E256L is a high speed memory. It must have a high
frequency bypass capacitor of approximately 0.1 µF connected
between V
CC
and V
SS,
using leads and traces that are as short
as possible. As with all high speed CMOS ICs, careful routing of
power, ground, and signals reduce circuit noise.
Hardware Protect
The CY14E256L offers hardware protection against inadvertent
STORE operation and SRAM WRITEs during low voltage condi-
tions. When V
CAP SWITCH
, all externally initiated STORE
operations and SRAM WRITEs are inhibited. AutoStore can be
completely disabled by tying VCC to ground and applying + 5V
to V
CAP
. This is the AutoStore Inhibit mode; in this mode,
STOREs are only initiated by explicit request using either the
software sequence or the HSB pin.
Low Average Active Power
CMOS technology provides the CY14E256L the benefit of
drawing significantly less current when it is cycled at times longer
than 50 ns.
Figure 4 shows the relationship between I
CC
and
READ or WRITE cycle time. Worst case current consumption is
shown for both CMOS and TTL input levels (commercial temper-
ature range, VCC = 5.5V, 100% duty cycle on chip enable). Only
standby current is drawn when the chip is disabled. The overall
average current drawn by the CY14E256L depends on the
following items:
■ The duty cycle of chip enable
■ The overall cycle rate for accesses
■ The ratio of READs to WRITEs
■ CMOS versus TTL input levels
■ The operating temperature
■ The V
CC
level
■ IO loading
Preventing Store
The STORE function is disabled by holding HSB high with a
driver capable of sourcing 30 mA at a V
OH
of at least 2.2V,
because it has to overpower the internal pull down device. This
device drives HSB LOW for 20
μs at the onset of a STORE.
When the CY14E256L is connected for AutoStore operation
(system V
CC
connected to V
CC
and a 68
μF capacitor on V
CAP
)
and V
CC
crosses V
SWITCH
on the way down, the CY14E256L
attempts to pull HSB LOW. If HSB does not actually get below
V
IL
, the part stops trying to pull HSB LOW and abort the STORE
attempt.
Figure 4. Current Versus Cycle Time (READ)
Figure 5. Current Versus Cycle Time (WRITE)
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